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Sneakiest primates have biggest brains

Monkeys and apes who are good at deceiving their peers also have the biggest brains relative to their body size. The finding backs the “Machiavellian intelligence” theory, which suggests the benefits of complex social skills fuelled the evolution of large primate brains.

Of all the terrestrial mammals, primates have by far the largest brains relative to their body size, with humans having the largest of all. The enlargement is almost exclusively in the neocortex, which makes up more than 80% of the mass of the human brain.

Large brains, despite being energetically costly, benefited primates because they conferred complex cognitive skills. But which skills were the priority – was it clever food-finding strategies that were most valuable, for example, or complex social skills?

Earlier studies have hinted that social abilities were the key. And now Richard Byrne and Nadia Corp, psychologists at St Andrews University in the UK, have found more direct evidence for this after studying records of primates deceiving each other for personal gain.

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Surreptitious mating

Deception amongst primates is well documented. Sometimes a female gorilla will mate with a male surreptitiously to avoid a beating from a more dominant male. Or monkeys might feign disinterest in tasty food so that others do not come and steal it.

Byrne has himself observed a young baboon dodging a reprimand from its mother by suddenly standing to attention and scanning the horizon, conning the entire troop into panicking about a possible rival group nearby. “We were rather shocked that baboons could do anything quite as subtle as that,” he says.

Now Byrne and Corp have studied a catalogue of observations of deceptive behaviours in wild primates from many researchers over several years up till 1990. They found that the frequency of deception in a species is directly proportional to the average volume of the animal’s neocortex.

Bush babies and lemurs, which have a relatively small neocortex, were among the least sneaky. The most tactically deceptive primates included macaques and the great apes – gorillas, chimpanzees, bonobos and orang-utans – which also have the largest neocortex.

Concept of dishonesty

That is consistent with the idea that natural selection favoured larger brains for sophisticated social interactions, among them tactical deception.

“I’m sure if we could have measured cooperative skill, we’d have found a similar result,” says Byrne. “Cooperation and outwitting are not opposed – they’re both about being socially subtle.”

However, it is still not clear whether primates are ever aware of being deceptive. They may have no concept of dishonesty, knowing simply from experience that these behaviours get the result they want.

Journal reference&colon; Proceedings of the Royal Society B (DOI&colon; 10.1098/rspb.2004.2780)